1. Field of the Invention
The present invention relates to a carrier for the electrostatic latent image developers, an electrostatic latent image developer and an image forming method used for developing an electrostatic latent image formed by electrophotography and electrostatic recording methods and the like.
Furthermore, the present invention relates to an image forming method executed by a image forming apparatus such as a digital printer, a digital copying machine and the like which handles an image as a digital signal.
2. Description of the Related Art
The methods to visualize the image information via the electrostatic latent image, such as electrophotography, have been utilized in various fields. In electrophotography, an electrostatic latent image is formed on a photosensitive material in the charging and the light exposure processes, and the electrostatic latent image is developed by a developer containing a toner, and visualized through the transfer and the fixing processes.
On the other hand, in the digital image forming apparatus, binary information of ON/OFF is given as two-dimensional information of predetermined place on the photosensitive material based on the character and image data. When a halftone image is recorded by using such method, an area modulation method using a halftone dot structure and multiple line structure has been conventionally adopted in various printers and copying machines of the digital electrophotographic method, since the algorithm is relatively simple and of low cost.
Furthermore, in the image forming apparatus which reproduces a multi-tone image by using the electrophotographic method, in particular, in a color image forming apparatus, there are two-component developers comprising a toner and a carrier, and one-component developers using a magnetic toner or the like singly, as the developer. The two-component developer has been widely used since the carrier fulfills the functions of stirring, transferring and charging of the developer, thus functions are separated in the developer, and stable charging characteristics and good controllability can be obtained.
Furthermore, as the developing method, a cascade method had been used before, but recently a magnetic brush method which uses a magnetic roll as a developer transfer carrying member has become the mainstream. As the two-component magnetic brush development method, a conductive magnetic brush (CMB) development method using a conductive carrier and an insulating magnetic brush (IMB) development method using an insulating carrier are known.
The IMB development is characterized by the relationship between the latent image potential and the image density on the photosensitive material is linear and the gradient is small, but on the other hand, the solid portion is not sufficiently filled and the edge effect is large. On the other hand, the CMB development has characteristics that it does not have the edge effect and the solid portion is sufficiently filled, in contrast to the insulating magnetic brush developing method, but the relationship between the latent image potential and the image density is abrupt and the gradient is large, and has such defects that carrier-over (migration of the carrier toward the photosensitive material) and brush marks due to the break of the latent image caused by a bias leak are easily caused.
These problems do not have much influence on visual image quality, when black and white images are formed by using only a black toner, and if the degree of the defects is soft. When color images are formed by superposing color toners, however, these problems will become fatal defects. In the black and white images, the above problems are caught only as a microscopic change in density, while in the color images, the above problems are caught as a microscopic change in hue, and noises having different colors will exist in the gradation image. Accordingly, the above problems have an extremely bad effect on the quality of the visual image, particularly of a color image.
There have been disclosed some methods to improve these points and obtain a conductive magnetic brush in which the edge effect is small, and carrier-over and brush marks are hardly caused.
For example, in Japanese Patent Application Publication (JP-B) No. 7-120,086, there is disclosed a carrier in which a core material having a relatively low electric resistance (hereinafter referred to as a "carrier core", or simply a "core") is coated with a resin having high resistance, thereby the electric resistance abruptly changes in some electric field, and it has high resistance in a low electric field, and low resistance in a high electric field. According to the disclosure thereof, since the latent image portion has a high electric field and the non-latent image portion has a low electric field, an excellent solid black printing can be obtained while carrier-over in the non-latent image portion is not caused. In this invention (according to the description in the examples and effects in the publication Japanese Patent Application Publication (JP-B) No. 7-120,086), however, it is presumed that the film thickness of the resin coating layer is considerably thin, and cores having low resistance are exposed partially. Hence, it is considered that the resistance is low in the high electric field due to this structure. Actually, as in the Comparative Examples described later, the electric resistance of a carrier whose core material is coated completely with a resin coating layer having a thick film thickness is high even in a high electric field, and a good solid image could not be obtained. With the partially coated carrier described above in which a part of the core material having low resistance is exposed, the electric charge moves easily via the exposed face, hence brush marks are easily caused in the latent image portion.
Furthermore, in Japanese Patent Application Laid-Open (JP-A) Nos. 61-107,257 and 61-130,959, there is disclosed a ferrite having relatively low electric resistance and having unevenness based on the primary particles on the surface. According to the disclosure thereof, the leak between electric charges having different polarity is suppressed, and brush marks are prevented because of the ferrite having such a minute unevenness. However, since it has a minute unevenness on the carrier surface, the contact area with the toner increases, resulting in a problem that the toner easily adheres, and its charge-imparting ability as a carrier is deteriorated with the lapse of time.
Furthermore, in Japanese Patent Application Laid-open (JP-A) No. 6-161,157, there is described a carrier defined by a ratio of the current value of the carrier core and the current value of the whole carrier coated with a resin, and hence the resolution, density of the solid image and the fine line reproducibility can all be satisfied at the same time. However, sufficient effect is not found in the prevention of the image defects in color images, in particular.
In addition, there is found no report in either example described above that the stability against the environmental changes is improved.
As described above, with regard to the CMB development, the improvement is not sufficient from the standpoint of the recent demand for the high image quality, including color images.